<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <!-- 引入vue -->
  <script type="text/javascript" src="../js/vue.js"></script>
</head>

<body>
  <!-- 
  监视属性watch:
  1.当监视数据的属性变化时，回调函数自动调用，进行相关的操作
  2.监视的属性必须存在，才能进行监视！！！
  3.监视的两种写法：
  (1) .new Vue 时传入watch配置
  (2) 通过vm.$watch监视
 -->
  <div id="root">
    <h2>今天天气{{info}}</h2>
    <button @click="changeWea">切换天气</button>
  </div>
  <script type="text/javascript">
    Vue.config.productionTip = false //阻止 vue 在启动时生成生产提示。、
    const vm = new Vue({
      el: '#root',
      data: {
        isHot: true
      },
      computed: {
        info() {
          return this.isHot ? '炎热' : '凉爽'
        }
      },
      methods: {
        changeWea() {
          this.isHot = !this.isHot
        }
      },
      /*  watch: {
         isHot: {
           immediate:true,
           // handler当isHot发生改变时会调用
           handler(newValue, oldValue) {
             console.log('ishot已修改', newValue, oldValue);
           }
         }
       } */
    })
    vm.$watch('isHot', {
      immediate: true,
      handler(newValue, oldValue) {
        console.log('ishot已修改', newValue, oldValue);
      }
    })
  </script>
</body>

</html>